Precision time technology has become central to many electronics applications and especially to distance and position determination. A wireless transmission error of one billionth of a second can create a location error of one foot. World time standard bodies have proposed optical clocks in space because natural continental plate drifting at 1 cm per day would be too unstable for femtosecond precision time applications. This technology has far reaching impact, and may affect many other disparate disciplines such as power grid energy efficiencies, communications, and time synchronized remote sensing (e.g., earthquakes) that rely upon precision time.
Sending a precise time has typically been accomplished by transmitting a precise timing marker such as a zero to one transition of a designated bit waveform of a train of bits with an annotated time of day stamp. Less common is the practice of specifying a prescribed “n” percent rise threshold of a digital bit or analog wave in order to obtain greater precision. Systems that have receiver clocks synchronizing with a transmitter's precision clock can obtain precision to about 1 percent of the signal's digital bit width.
Multiplexing is the concept of several transmitters sending information simultaneously over a single communications channel. This allows several users to share a single band of radio or optic frequencies. Frequency Division Multiplexing, Time-Division Multiplexing, and Code Division Multiple Access, as shown in FIG. 1, are major ways of multiplexing separate signals.
Time-division multiplexing (TDM), illustrated at 102 in FIG. 1, is a type of digital (or sometimes analog) multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns (in time) on the channel. Frequency division multiplexing (FDM) illustrated at 104 in FIG. 1 involves assigning non-overlapping frequency ranges to different signals or to each user of a medium, such that the signals may be transmitted simultaneously. Code division multiple access (CDMA) shown at 106 in FIG. 1 attaches different identification tags to individual tributary signal segments in order for multiple transmitters “speak” independently and simultaneously on the same shared frequency band, yet remain distinguishable and understandable by receivers. By analogously adding prefixes, “McFries, McRibs, and McMuffin” can be audibly distinguished within a family from “iPod, iPhone, iPad, and iTunes”. CDMA is a multiplexing channel access method used by various radio communication technologies. It should not be confused with CDMA2000 (3G evolution) and WCDMA (3G GSM carriers), which are specific cell phone technology standards and which are often referred to as simply CDMA.
Multiplexing data compressors merge transmission waveforms by making approximations to increase and discern repetitiveness to reduce signal content. For example, such compressors expect the clock waves on clock and data pairs to be equidistant with no requirement that transition rise times must be precisely replicated; i.e., they address only zero or one states.
With such multiplexing, receiver nodes can adequately reconstruct tributary signals, but designers must often carefully plan transmissions so that timing markers do not collide with other timing markers or other signal element waveforms.
There is also concern for efficient practical use of any given transmission signal. Time division multiplexing (TDM) must widen its tolerances so that any tributary signal time interval, taking its turn, will not overlap with others. Similarly, frequency division multiplexing (FDM) systems must use a guard-band between adjacent frequency channels, due to the unpredictable Doppler shift of the signal spectrum as a user platform moves. Since CDMA appends identification to transmitted signal element instances, it is inherently inefficient.
Typically, transmission signals with timing markers are not 100% filled with timing markers and data. But any unfilled empty space in terms of random idle periods or partially filled packet headers cannot be easily used by other possible contributing tributary signals because their signal element waveforms may unavoidably overlap, overextend, overwrite, or otherwise collide. Dynamic biasing or statistical multiplexing can sometimes be used to adjust near term load balancing, but this is very difficult or near impossible when precision timing markers may unpredictably appear.
Often scenarios for precision timing markers require only sparse signals. For example, synchronizing precision clocks over great distances may need infrequent but very precise updates. Hence, the industry term 1 PPS pulse per second refers to a timing marker that occurs once a second but is nonetheless extremely precise. In order to achieve greater and greater precision, the above methods must correspondingly increase signal frequency and thus channel capacity. This implies much waste on this type of signal channel.
Although the above methods may permit the use of error detection and correction for information content, there is little provision for error detection and correction for individual precision timing marker bit positions if noise or lightning-caused gaps should occur.
It is further noted that when transmitters or receivers may be moving relative to each other or the transmission must travel through atmospheric distortions, travel time delay corrections may be necessary. There can also be Einsteinian special and general relativity effects which are induced by high spacecraft speeds or gravity gradients which can introduce errors as large as a microsecond. Still further, aircraft motion can introduce errors in the form of Doppler radio frequency shifts, misleading radio frequency variations, or a wobbling time reference.
Accordingly, known transmission systems may further have precision timing markers that sometimes use two or more radio frequencies to allow downstream receivers to discern signal travel delay differences and thus make corrections to account for sources of distortion such as the ionosphere. Notwithstanding, local, national, or international radio frequency congestion is an increasing problem and obtaining an allocation of frequency can be difficult or impossible. Also, the potential for existing GPS spacecraft positioning-navigation-and-timing systems to fail servicing location finders has long been acknowledged by the Air Force and many government agencies. The Air Force, Navy, Army, DARPA, Federal Aviation Administration, Homeland Security, and Coast Guard are actively funding alternatives which would be used whenever this system fails any local region. Primary obstacles are expensive infrastructures having new backup transmitters and new receivers as well as the unavailability of radio frequencies which are rationed by the FCC. These transmitters need to send their exact location and precise time of transmission, so that user location finders can determine the signal travel time. Precision timing markers are thus key components in those transmissions.
Accordingly, there exists a need to provide methods and apparatus for improving transmission platforms, corresponding receiver platforms, and transmission signals which use precise timing markers.